The present invention relates to a through type high withstand voltage ceramic capacitor, in particular, relates to such a capacitor for the use in the noise filter in a high power and high frequency apparatus, like a microwave oven, a broadcasting transmitter and/or an X-ray generator.
In a high frequency and high power apparatus operating in the VHF band or UHF band, the noise superposed on a commercial power line must be prevented. For that purpose, a noise filter, which is actually a low-pass filter is inserted in a power supply line of an apparatus.
FIG. 1A shows the circuit diagram of a noise filter inserted in a power supply line of a heater of a magnetron in a microwave oven. In FIG. 1A, the capacitors C.sub.1 and C.sub.2, and the inductors L.sub.1 and L.sub.2 compose a noise filter, the reference numeral 100 is a conductive filter housing, 102 is a magnetron, 104 is a heater of the magnetron 102, and 106 is an anode of the magnetron 102, and that anode is grounded.
The capacitors C.sub.1 and C.sub.2 must have a high withstand voltage, since a magnetron has a high operational voltage between an anode electrode and a heater. Further, those capacitors must have excellent temperature characteristics, since those capacitors are repetitively heated to a high temperature in a microwave oven.
The present invention relates to the improved structure of a capacitor for the use in for instance the noise filter in a microwave oven.
The capacitor for that purpose is a through capacitor, and two separate capacitors, or a twin capacitor which has two capacitors in a single housing is utilized for that purpose.
FIG. 1B shows the example of a prior twin through capacitor for a noise filter, FIG. 1C is a cross sectional view of the capacitor of FIG. 1B, and FIG. 1D is the cross sectional view of the noise filter utilizing a twin capacitor of FIGS. 1B and 1C.
In those figure, an elliptic ceramic body 1 has a pair of holes 2 and 3 in the thick vertical direction of the ceramic body 1. On the upper surface of the ceramic body 1, a pair of separated electrodes 4 and 5 having corresponding holes are attached, and on the lower surface of the ceramic body 1 a common electrode 6 is attached. The rectangular ground conductor 7 has a plate 7c and a elevated portion 7a. The plate 7c has four holes 7c-1 through 7c-4 for fixing the twin capacitor to the filter housing, and the elevated portion 7a has a pair of holes 9 and 10 which correspond to the holes 2 and 3 on the ceramic body 1, and said elevated portion 7a has also a plurality of small holes 7b along the peripheral line of the elevated portion 7c. Those small holes 7b serve to flow an insulating means as described later. The common electrode 6 is fixed on the elevated portion 7a of the ground conductor 7 so that the holes 9 and 10 coincide with the holes 2 and 3, respectively, and the small holes 7b are positioned outside of the ceramic body 1. The pair of elongated through conductors 11 and 12 are inserted in the holes 2 and 3, and the holes 9 and 10, respectively, so that those conductors 11 and 12 do not electrically contact with the common electrode 6. In order to assure the insulation between the common electrode 6 and the conductor rods 11 and 12, those rods 11 and 12 are covered with flexible plastics tubes (insulation tubes) 15 and 16, respectively. The caps 13 and 14 are put on the upper portion of the conductive rods 11 and 12, respectively, and those caps 13 and 14 are soldered to both the conductive rods 11 and 12, and the electrodes 4 and 5, respectively, in order to assure the electrical contact between the rods 11 and 12, and the electrodes 4 and 5, respectively. Those caps 13 and 14 also have elevated portions which have a plurality of small holes 13a and 14a, respectively, along the each peripheral line. The hollow elliptic cylindrical plastic cover 8 is attached under the ground conductor 7 so that said cover 8 encloses the rods 11 and 12 with the tubes 15 and 16, respectively.
The insulation filler 17, which is for instance epoxy resin, covers the portion of the bottom of the cover 8, and the periphery of the ceramic body 1, the caps 13 and 14 and the rods 11 and 12, as shown in FIG. 1C. In injecting the insulation filler 17, the capacitor body is covered with the cover 18, and the filler 17 is injected in the capacitor from the bottom of the cover 8. The injected insulation filler is injected into the capacitor through the small holes 7b provided on the ground conductor 7, and the small holes 13a and 14a provided on the caps 13 and 14, and thus, the area inside of the cover 18 is filled with the insulation filler 17. After the filler thus injected is hardened, the cover 18 is removed, and the twin capacitor is completed, and the insulation and the protection of the capacitor from the moisture are assured by the injected epoxy resin.
However, a prior twin capacitor as described above has the disadvantages as follows.
The first disadvantage is that the life time of a capacitor is rather short when utilized in a microwave oven. The reason for that short life time comes from the substantially elliptic shape of the filled insulator 17. Because of the elliptic insulator 17, the distribution of the stress generated in the insulator 17 is not uniform. That nonuniform stress is generated in the insulator 17 when the insulator 17 is cooled and hardened in the manufacturing stage, and/or the capacitor is repetitively heated in a microwave oven. In particular, in the case of a microwave oven with a steam oven, the capacitor is disposed in the atmosphere with high temperature and high moisture repetitively. The nonuniform stress in the insulator 17 generated by the temperature change causes gaps and/or cracks between the insulator 17 and the capacitor components (ground conductor 7, cover 8, and/or conductor rods 11 and 12 et al). If gaps and/or cracks are generated, the electromagnetic fields in the cracks are increased, and the withstand voltage and/or the voltage at which an arc is generated is decreased.
Another disadvantage of the prior twin type capacitor is that the particular design consideration is necessary for the conductor rods 11 and 12, the insulator 17 and the cover 8 because of the nonuniform distribution of the stress in the insulator 17.
Another disadvantage of the prior twin type capacitor is that the size of the capacitor must be large in order to assure the preferable withstand voltage in spite of the generation of gaps and/or cracks.
It should be appreciated that said nonuniform stress in the insulator comes from the substantially elliptic shape of the insulator, and said elliptic shape comes from the twin capacitor. In case of a single capacitor, the whole body is circular, and the stress in the body is uniform, and thus, the disadvantage described above does not exist.